(core) Grace period and delete-only mode when exceeding row limit

Summary:
Builds upon https://phab.getgrist.com/D3328

- Add HomeDB column `Document.gracePeriodStart`
- When the row count moves above the limit, set it to the current date. When it moves below, set it to null.
- Add DataLimitStatus type indicating if the document is approaching the limit, is in a grace period, or is in delete only mode if the grace period started at least 14 days ago. Compute it in ActiveDoc and send it to client when opening.
- Only allow certain user actions when in delete-only mode.

Follow-up tasks related to this diff:

- When DataLimitStatus in the client is non-empty, show a banner to the appropriate users.
- Only send DataLimitStatus to users with the appropriate access. There's no risk landing this now since real users will only see null until free team sites are released.
- Update DataLimitStatus immediately in the client when it changes, e.g. when user actions are applied or the product is changed. Right now it's only sent when the document loads.
- Update row limit, grace period start, and data limit status in ActiveDoc when the product changes, i.e. the user upgrades/downgrades.
- Account for data size when computing data limit status, not just row counts.

See also the tasks mentioned in https://phab.getgrist.com/D3331

Test Plan: Extended FreeTeam nbrowser test, testing the 4 statuses.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3331

app/server/lib/requestUtils.ts

@@ -18,9 +18,10 @@ export const TEST_HTTPS_OFFSET = process.env.GRIST_TEST_HTTPS_OFFSET ?
   parseInt(process.env.GRIST_TEST_HTTPS_OFFSET, 10) : undefined;
 
 // Database fields that we permit in entities but don't want to cross the api.
-const INTERNAL_FIELDS = new Set(['apiKey', 'billingAccountId', 'firstLoginAt', 'filteredOut', 'ownerId',
-                                 'stripeCustomerId', 'stripeSubscriptionId', 'stripePlanId',
-                                 'stripeProductId', 'userId', 'isFirstTimeUser', 'allowGoogleLogin']);
+const INTERNAL_FIELDS = new Set([
+  'apiKey', 'billingAccountId', 'firstLoginAt', 'filteredOut', 'ownerId', 'gracePeriodStart', 'stripeCustomerId',
+  'stripeSubscriptionId', 'stripePlanId', 'stripeProductId', 'userId', 'isFirstTimeUser', 'allowGoogleLogin',
+]);
 
 /**
  * Adapt a home-server or doc-worker URL to match the hostname in the request URL. For custom